For more than a century (1825 to 1950) "Ball Mills" were used to grind and disperse pigment for paint, coatings, ink, and other pigment-vehicle fluids. Ball mills use steel, stone, or ceramic balls. Some disadvantages to ball mills are that they require a long dispersion time, they are hard to clean, and they take up large amounts of space. Not only are ball mills generally much larger than subsequent generation of grinding mills, but ball mills are generally supported in a horizontal configuration because the vertical configuration requires more floor space below the structure than the horizontal configuration. A more "modern" concern is an environmental one; ball mills create heavy noise pollution problems. This is primarily due to the "balls" banging about the mill, and the required heavy rotating machinery.
In the later 1950's, the first "Media Mills" were developed. Media mills, also known as "sand mills" or "sand grinders", use very small diameter grinding media, on the order of 20-40 mesh Ottawa type sand. This takes advantage of the large surface area per unit weight which cannot be used effectively in ball mills. The basic media mill structure consists of a vertical cylinder with a rotating shaft through its center, two or more flat impellers on a shaft, and a pump at the bottom which forces a premixed pigment-vehicle fluid paste through the milling zone and out through a discharge screen to retain the sand. Fluid is discharged into drums or pumped on for additional processing.
A specific example is a Moorehouse-Cowles mill. The 12-50 model uses a twelve inch diameter cylinder with ten inch diameter discs on a rotatable shaft and a 50 HP motor. Moorehous-Cowles manufacturers media mills with barrel diameters approximately 6, 8, 10 and 12 inches; Chicago Boiler Corp. (CBC) has similar sizes available. As the size of the barrel increases in diameter, the amount of output of dispersed pigment increases. However, the larger the barrel, the more horse power is required to drive the mill to produce a specific quantity of dispersed pigment. For instance, a six inch mill from Chicago Boiler may be powered by 15 HP electric motor whereas a twelve inch mill will need a 40-50 HP motor.
It is believed that in a single cylinder mill, grinding is mostly accomplished through impact of the mixture on the cylinder walls and shearing forces produced by velocity differences between layers of fluid. Dispersion is mostly the result of stacked vertices within the cylinder and the shearing forces. Rotating the impellers causes sand-paste mixture near their surfaces to rotate faster than the layers farther away. The layers closest to the top-side of an impeller are pulled from the shaft region and pushed toward the cylinder wall by the centrifugal force produced when the shaft is rotated. These layers are then forced up the side of the cylinder wall. Simultaneously, layers closest to the bottom-side of an impeller are also pulled from the shaft region and pushed towards the cylinder walls. In contrast to the layers closest to the top-side of an impeller, the layers closest to the bottom-side of an impeller will be forced down the walls of the cylinder. Thus, in an area between the top-side of an impeller and the bottom-side of an adjacent impeller, the fluid layers collide and are forced back toward the shaft region. This results in counter-clockwise rolled layers, i.e. a counter-clockwise vortex near the top-side of an impeller and a clockwise vortex near the bottom-side of an impeller.
The difference in velocity of adjacent layers produces the required shearing action for further grinding and dispersion. Pigment aggregates between two sand particles in adjacent layers are subjected to a compressive force in one direction, which resolves into a shearing stress at a different angle. The maximum shear stress at the impeller surface is, roughly, a function of the pigment cross sectional area, given a constant media (sand) cross-section. Allowing that the shear stress in the overall mixture is approximately (1/2 psi), an approximation for the maximum shear stress at the impeller's surface reduces to: (1/2 psi) times the ratio of sand cross-sectional area to pigment cross sectional area.
The consistency of the sand-paste must be carefully adjusted. The fluid-paste must be viscous enough to be moved by impellers, but excessive viscosity reduces sand movement and milling action. Too low a consistency results in poor circulation, sand skidding, and wear on impellers. A machine can be jacketed for control of temperature during grinding. The machine may be cleaned by running solvent through it. However, sand costs are relatively cheap, so the sand may be discarded to avoid cleaning a hard to clean color out of media. Using sufficient premixers to ensure continuous flow of paste to the mill, the sand grinder can be used for automatic continuous operation.
While the standard single cylinder has been a significant improvement over standard ball mills and has been adequate for most purposes, it does have its drawbacks. Milling time can be long, especially for larger diameter particulates. Some matter, for instance carbon blacks, can only be milled using steel ball mills. Steel ball mills are not an acceptable milling process in today's environment due to the disadvantages previously mentioned. Additionally, standard gear pumps used in sand mills are subject to severe wear if media gets into the pumping gear and pump system.
It would be useful to develop an improved dispersing chamber with improved grinding efficiency over that of standard media mills. A chamber that would reduce the cycle time of hard to disperse/grind pigment, or grind particulates not heretofore possible to grind with a media mill, would be a significant improvement over existing dispersing chambers.